Hard drive platters are formed from various magnetic materials applied to a substrate disc. Traditional hard drives use magnetic materials that can be read and written by a magnetic read/write head. However, other technologies have been suggested. In particular HAMR (Heat Assisted Magnetic Recording) materials use a thermally assisted write mechanism. This allows magnetic materials with relatively high coercivity to be used, with smaller bit sizes and higher storage density. In order to write to the disk, a laser beam is used to heat the magnetic material above the Curie temperature. The coercivity drops above this temperature, and a magnetic write field can be used to write data onto the disk. HAMR materials are a good candidate for future advances in hard drive technology.
In testing of hard drive platters, researchers probe the magnetic properties of the platter or a material sample using varying magnetic fields. In one method, a sample or platter is positioned in a magnetic field which can be ramped over time. As the field is ramped, Magneto-Optical Kerr Effect (“MOKE”) measurements can be made. This involves introducing a polarised light beam onto the sample surface. The magnetized sample surface causes alterations in the polarisation and/or ellipticity of the reflected beam. The MOKE is well understood and need not be discussed in detail in this specification. However, a typical data set is illustrated in the graph of FIG. 1, where the vertical axis is magnetisation M, measured by MOKE techniques, and the horizontal axis is applied magnetic field H. This hysteresis loop is typical of magnetic materials used in hard drive platters.
In existing MOKE equipment, the polar Kerr effect is often used. This requires the magnetic field to be perpendicular to the sample surface and parallel to the plane of incidence. A typical apparatus is illustrated in FIG. 2. The apparatus includes a first magnetic pole 1 and second magnetic pole 2. Typically electromagnets are used, but the coils are not shown in FIG. 2. The first magnetic pole 1 has an aperture 3 to allow a polarised light beam 4 to be introduced. Generally the light beam is introduced perpendicular to the sample surface 5. In order not to influence the performance of the magnet, the aperture is generally just a few millimetres in diameter. The reflected laser beam 6 passes out through the same aperture and is separated from the incoming beam by a beam splitter 7. Various further optical components are also used, as will be understood by the skilled reader. In particular, a light source, light detector, polariser, analyser, modulator, lenses etc may be used as necessary for the particular application.
In order to measure the latest hard drive materials, higher magnetic fields are required. This leads to difficulties. While copper-wound electromagnets can be used up to fields of perhaps 3-5 tesla, higher fields require electromagnets wound with superconducting wire. Superconducting magnets are lossless in DC mode. However, when the magnetic field is ramped over time the superconducting coils produce heat. The required electrical ramping of the magnetic field over time therefore causes heat losses and inefficiency. At fast ramping rates, this would require additional cooling and more superconducting wire operating at lower current density. This would significantly raise costs. Further, the faster the ramping rate the higher the required voltage. Faster ramping requires high voltage power supplies that are not readily available in a form suitable for powering superconducting magnets.
These factors limit the speed at which the magnetic field can be ramped with acceptable performance, with a full four quadrant ramp (i.e. to define the full hysteresis loop of FIG. 1) taking about 1.5 to 5 minutes at 6-7 tesla. As the laser spot used is typically around 2-3 mm in diameter, it therefore takes several minutes to measure a very small area on the sample surface. Further, upgrading the speed capability of existing equipment would require either expensive new magnets or an expensive rebuild of the existing magnets.
In addition to the above problems, the ramping magnetic field creates time-varying fringe fields that can adversely affect any nearby equipment or experiments.
Arora et al (Arora, Ghosh and Sugunakar “A mirror based polar magneto-optical Kerr effect spectroscopy arrangement” Review of Scientific Instruments 82 123903 (2011)) suggest a polar MOKE apparatus in which a mirror is used to redirect the light beam onto a sample, such that an aperture in the magnet pole is not required. However, this system still suffers from the other drawbacks discussed above.
Reference to any prior art in this specification does not constitute an admission that such prior art forms part of the common general knowledge.
It is an object of the invention to provide a magneto-optical testing apparatus that addresses or at least ameliorates one or more of the above problems, or at least to provide the public with a useful choice.